This invention relates in general to unsegmented monochromatic cathode ray tube image displays, and is particularly concerned with means for enhancing the brightness of such displays, without loss of resolution.
High brightness, resolution and contrast are requisites for the discernibility of monochromatic cathode ray tube information displays. These requisites are difficult to achieve because the enhancement of one of the factors may result in the deterioration of another. For example, attempting to increase the image brightness by increasing cathode ray tube beam current results in loss of resolution as beam diameter, and hence beam spot size (or line width), is almost directly proportional to beam current. An increase in beam current from one milliampere to two milliamperes in a standard bipotential gun, for example, will normally increase spot size by forty percent. And the larger the beam spot size, the lower the resolution. Another example: Image contrast is normally enhanced by installing a neutral density filter over the glass face panel of the cathode ray tube, or by introducing neutral density filter elements into the composition of the face panel. However, any increase in contrast by this means results in lower image brightness. The application of a filter of twenty percent density, for example, will result in a twenty percent brightness loss. A loss of this magnitude in conventional cathode ray tubes can wash out an image under bright ambient light conditions.
The electron gun according to one embodiment of the present invention represents a new and useful improvement of the electron gun having an extended field main focus lens described and claimed in U.S. Pat. No. 3,995,194 to Blacker et al, assigned to the present assignee, and incorporated herein by reference. The novel focus lens set forth by the '194 patent takes advantage of the low aberrations produced by the extended field lens described and claimed in U.S. Pat. No. 3,895,253 to Schwartz et al, also of common ownership herewith. The extended field provides reduced beam spot size over a wide range of beam currents, and thus offers markedly improved picture resolution in all screen sizes.
In U.S. Pat. No. 3,696,261, Miyaoka discloses a color cathode ray tube of increased brightness having a group of beams for each color element. The screen is segmented in that the red, green and blue phosphors are in discrete sets, or arrays, over the area of the screen. In one embodiment, six beams are generated, with paired beams having a common landing point on a corresponding red, green or blue stripe after passing through a color selection electrode. The pairs of beams pass through a common optical center of the main focus lens. In another aspect, nine beams are generated to provide trios of beams having common landing points on corresponding color stripes. The beams, whether in pairs or trios, are oriented in a vertical plane.